In a radio communication, improvement of frequency utilization efficiency for increasing a capacity of communication by using a limited frequency source has been desired. There has been proposed a Multiple Input Multiple Output (MIMO) system as a technique for improving the frequency utilization efficiency. The MIMO system configures a space multiplex channel on substantially the same frequency band at substantially the same time by a plurality of transmit antennas and a plurality of receive antennas, and improves an information transmitted rate.
There is QR decomposition and M-algorithm (QRM-MLD) as a method for separating transmitted signals from a transmitter on a receiver side. By using the QRM-MLD, a channel matrix between the transmitter and the receiver is subjected to QR decomposition, and received signals are layered based on the result. Accordingly, an operation amount of the receiver is reduced by reducing signal candidate points if the signals without an interference signal component, that is, the signals in the last layer after being layered are sequentially used.
If three transmit antennas and three receive antennas are used, a channel matrix H is expressed as the following formula (1). In this case, elements h1,1, h2,1, and h3,1 indicate characteristics for receiving a transmitted signal from a first transmit antenna by first, second, and third receive antennas, respectively.
                    H        =                  [                                                                      h                                      1                    ,                    1                                                                                                h                                      1                    ,                    2                                                                                                h                                      1                    ,                    3                                                                                                                        h                                      2                    ,                    1                                                                                                h                                      2                    ,                    2                                                                                                h                                      2                    ,                    3                                                                                                                        h                                      3                    ,                    1                                                                                                h                                      3                    ,                    2                                                                                                h                                      3                    ,                    3                                                                                ]                                    (        1        )            
The QR decomposition of the channel matrix H may be expressed as the following formula (2).
                    H        =                  Q          ⁡                      [                                                                                r                                          1                      ,                      1                                                                                                            r                                          1                      ,                      2                                                                                                            r                                          1                      ,                      3                                                                                                                    0                                                                      r                                          2                      ,                      2                                                                                                            r                                          2                      ,                      3                                                                                                                    0                                                  0                                                                      r                                          3                      ,                      3                                                                                            ]                                              (        2        )            
If each of the received signals is multiplied by a complex conjugate transposition of a unitary matrix generated by the QR decomposition and the received signals are layered in such a way that the transmitted signal component exists mainly in the first row, a received signal vector after layering yn may be expressed as the following formula (3). Here, x1, x2, and x3 indicate transmitted signals of the first, second, and third transmit antennas, respectively.
                    y        =                                            [                                                                                          r                                              1                        ,                        1                                                                                                                        r                                              1                        ,                        2                                                                                                                        r                                              1                        ,                        3                                                                                                                                  0                                                                              r                                              2                        ,                        2                                                                                                                        r                                              2                        ,                        3                                                                                                                                  0                                                        0                                                                              r                                              3                        ,                        3                                                                                                        ]                        ⁡                          [                                                                                          x                      1                                                                                                                                  x                      2                                                                                                                                  x                      3                                                                                  ]                                +          II                                    (        3        )            
For example, if the transmitted signals x1, x2, and x3 are 64-QAM signals, the signal r3,3×x3 in the last row where there is no interference component is compared to 64 replicas of the signal x3 to select M pieces of replicas with a higher likelihood, so that the signal candidate points are reduced. After that, the signal r2,2×x2 is compared to the signal r2,3×x3, and 64 replicas of the signal x2 are compared to the replicas of M pieces of the signal candidate points of the signal x3 to select M pieces of replicas with a higher likelihood, so that the signal candidate points are reduced. Similarly, the signal r1,1×x1, r1,2×x2, and r1,3×x3 in the first row are compared, and 64 replicas of the signal x1 are compared to M pieces of the signal candidate points of the signals x2 and x1 respectively to select a signal candidate point with a higher likelihood.
There has been a proposal of a technique for performing the QR decomposition on a channel matrix, linear-filtering a corresponding received sequence, generating a transformation received sequence with respect to each subcarrier belonging to each group of subcarrier, and for refining candidates of a transmitted sequence in which space orders are rearranged, and outputting a final estimation result based on a candidate with the smallest likelihood metric as a temporary estimation result out of the candidates (for example, see Japanese Laid-open Patent Publication No. 2009-55216).
Moreover, there has been a proposal of a technique that a signal detection device refines the candidates of the transmitted sequence in which the space orders are rearranged based on an order of candidate signal point selection, and based on a result of the refined candidates of the transmitted sequence, outputting the candidate with the smallest likelihood metric as a temporary estimation result (for example, see Japanese Laid-open Patent Publication No. 2009-55217).
Furthermore, there has been a proposal of a technique that a received device calculates a new likelihood value as a next rank of the smallest likelihood value based on the rank of the likelihood value that corresponds to a combination of signal points, for deciding a smallest likelihood value as the smallest value among the likelihood values that include the new likelihood value and exclude the smallest likelihood value, and for selecting the combination of signal points that corresponds to the smallest likelihood values as a transmitted symbol candidate in each row (for example, see Japanese Laid-open Patent Publication No. 2009-141768).